In the past, a control system of an internal combustion engine which is provided with an air-fuel ratio sensor in an exhaust passage of the internal combustion engine, and controls an amount of fuel fed to the internal combustion engine based on the output of the air-fuel ratio sensor, has been widely known (for example, see PLTs 1 to 4).
In such a control system, an upstream side catalyst and downstream side catalyst which are provided in the exhaust passage and have oxygen storage abilities are used. A catalyst having an oxygen storage ability can purify unburned gas (HC, CO, etc.) or NOX, etc. in the exhaust gas flowing into the catalyst, when the oxygen storage amount is a suitable amount between an upper limit storage amount and a lower limit storage amount. That is, if exhaust gas of an air-fuel ratio richer than a stoichiometric air-fuel ratio (below, also called a “rich air-fuel ratio”) flows into the catalyst, the unburned gas in the exhaust gas is oxidized and purified by the oxygen stored in the catalyst. Conversely, if exhaust gas of an air-fuel ratio leaner than the stoichiometric air-fuel ratio (below, also called a “lean air-fuel ratio”) flows into the catalyst, the oxygen in the exhaust gas is stored in the catalyst. Due to this, the surface of the catalyst becomes an oxygen deficient state and, along with this, NOX in the exhaust gas is reduced and purified. As a result, the catalyst can purify exhaust gas regardless of the air-fuel ratio of the exhaust gas flowing into the catalyst so long as the oxygen storage amount is a suitable amount.
Therefore, in such a control system, to maintain the oxygen storage amount at the upstream side catalyst at a suitable amount, an air-fuel ratio sensor is provided at the upstream side, in the direction of flow of exhaust, from the upstream side catalyst, and an oxygen sensor is provided at the downstream side, in the direction of flow of exhaust, from the upstream side catalyst and at the upstream side, in the direction of flow of exhaust, from the downstream side catalyst. Using these sensors, the control system performs feedback control, based on the output of the upstream side air-fuel ratio sensor, so that the output current of this air-fuel ratio sensor becomes a target value corresponding to the target air-fuel ratio. In addition, the control system adjusts the target value of the upstream side air-fuel ratio sensor, based on the output of the downstream side oxygen sensor.
For example, in the control system described in PLT 1, when the output voltage of the downstream side oxygen sensor is a high side threshold value or more and the state of the upstream side catalyst is an oxygen deficient state, the target air-fuel ratio of the exhaust gas flowing into the upstream side catalyst is set to the lean air-fuel ratio. Conversely, when the output voltage of the downstream side oxygen sensor is at the low side threshold value or less and the state of the upstream side catalyst is an oxygen excess state, the target air-fuel ratio is set to the rich air-fuel ratio. According to PLT 1, due to this, when in the oxygen deficient state or oxygen excess state, it is possible to return the state of the catalyst quickly to a state in the middle of these two states (that is, state where catalyst stores a suitable amount of oxygen).
In addition, in the above control system, if the output voltage of the downstream side oxygen sensor is between the high side threshold value and the low side threshold value, when the output voltage of the oxygen sensor is in an increasing trend, the target air-fuel ratio is set to the lean air-fuel ratio. Conversely, when the output voltage of the oxygen sensor is in a decreasing trend, the target air-fuel ratio is set to the rich air-fuel ratio. According to PLT 1, due to this, it is considered that the state of the upstream side catalyst can be prevented in advance from becoming an oxygen deficient state or oxygen excess state.